Fix volumes search by metadata

The metadata parameter is urlencoded (to string) by cinderclient
but isn't decoded back to dict by the API.

The metadata parameter needs to be converted back to dict
to be of any use by the filters parameter of get_all().

The original implementation has been done by:
Mathieu Gagne <mgagne@iweb.com>

Fixes: bug #1195015

Change-Id: I19d7d386afddddc067e9f4ef86967c9b72d9e530
Merge "Fix volumes search by metadata"